carson



J. R. CARSON.

DISTORTION CORRECTING CIRCUIT.

APPLICATION man JULY 3.19 s.

Patented Sept. 9, 1919.

3 SHEETSSHEET 1.

IN VEN TOR.

A TTORNEY J. R. CARSGN.

DISTORTION CORRECTING CIRCUIT.

APPLICATION FILED JULY 3.!913.

mmmww Patentedsept. 9,1919.

3 SHEETS-SHEET 2.

IN V EN TOR.

@[MWRWMW BY Z 5 A TTORNE Y J. R. CARSON. DISTORTION CORRECTING CIRCUIT.-

APPLICATION FILED JULY 3.191s.

1,1 11; PatentedSept. 9,1919

3 SHEETS-SHEET s.

ATTORNEY UNITED STATES PATENT OFFICE.

JOHN R. CARSON, OF- NEW YORK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE ANDTELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

DISTOETION OORRECTING CIRCUIT.

. transmitting electrical energy and is especially applicable tosignaling systems in which it is desired to correct the wave form ofsignaling impulses which has been destroyed by the distorting effect ofthe transmission line. lln ocean cables, for instance, the transmittingvoltage at the sending end mav be represented by a curve of rectangularsquare topped form while the wave received atv the distant end is a veryflattened curve which requires a relatively long time to build up to itssteady value. For receiving purposes it is important to have an arrivalcurve whose wave form approximates that of the voltage impressed on thesending end. It is the purpose of the present invention to provide "adistortion correcting network capable of reforming the arrival curve incases of the kind above indicated. The invention will be more fullydescribed in connection with the accompanying drawings in which Figure 1illustrates diagrammatically one embodiment thereof; Fig. 2 is a detailof the amplifier circuits represented by the rectangles in Figs. 1 and5;. Fig. 3 is a set of curves representing a typical arrival curve andits derivatives; Fig. 4 is a curve indicating the form of arrival curvewhich may be derived by the employment of this invention and Fig. 5illustrates a modification of the arrangement shown in Fig. 1.

The invention, includes an arrangement embodying correcting networks per86 and amplifiers, preferably of the thermionic type, which may bearranged to introduce no distortion of their own. The amplifiers, in thepresent case, perform. the very important function of separating thedifferent sections of correcting network so-that one does not react uponitsvneighbor and the value of the elements in the networks may,

therefore, be readily calculated to give the desired result withouthaving to estimate I the modifying efiect of one network upon another. nthe process of distortion correction is'attended by diminution in thestrength of the received signal or when it is desired to increase theintensity thereof Specification of LcttcraPatent. Patented Sept, 9,1919, Application filed July a, me. Berta! no. am.

cause of its slow rate 0 building up. If a current of this form flowsthrough an inductive device such as the windings of a transformer or a.plain inductance coil, the voltage developed in said device will be thefirst derivative of the arrival current which wlll have a wave formapproximately as indicated at 2 in Fig. 3,-the derivative representingthe time rate of change of the arrival curve. Similarly if a current ofthe form of derivative 2 flow through an inductance coil the voltageacross the terminals of said coil will be the second derivative of thearrival current which has a wave form indicated at 3* in Fig. 3.Continuing by the same process successive derivatives may be obtained,the third and fourth being indicated at 4 and 5 respectively in Fig. 3.It Will be observed that the derivatives have a sharper and sharper risein value which is one of the" characteristics essential for goodrecording. 0n the other hand the curves of the successive derivativesfall ofi' more and more rapidly asindicated in the drawing and the bestcurrent for receiving purposes should have not only a sharp rise but asustained tail. It will be understood that the successive derivatives donot normally all have the same maximum value as shown but are soindicated merely to compare their characteristics conveniently. It

1s possible, however, by the use of amplifiers to give any particularderivative any desired value and it will be obvious from a considerationof the curves shown in Fig. 3

that by giving the various derivatives the proper value and combiningthem in a sinlot A curve 2 of Fig. 5. Consequently the voltnecesslty ofa metallic connection between K the line and said amplifiers. Owin tothe fact that the input circuits of amplifiers of the vacuum tube typeare of substantially.

infinite impedance, the secondary of transformer 13 is to all intentsand purposes open circuited, and the voltage developed across itsterminals unaffected by the presence of the amplifiers. Under suchconditions it may be shown that the voltage developed across theterminals of either primary or secondary of the transformer 13approximates the first derivative of the arrival curve of the cableitself, provided the inductance of the primary is relatively small, thisfirst derivative being of the form illustrated in age impressedin theinput circuits of amtional to the first derivative of the arrlval curveitself.

Across the circuit 11, ,12 is connected a 5 series of resistances 1' 1",1' 1', '1", 0", and

. voltage drop across resistance 1" is similarly proportional to saidfirst derivative.

The input circuit of amplifier A is connected by circuit-17 to'the-highside ofautocircuit of amplifier A retain the arrangement as shown sincea substantial voltage step u is, obtained by the inductive action of t eautotransformer. This is particularly advantageous because theinductance in the output circuit of amplifier A should be very smallcompared with the internal resistance of the tube if the voltage dropacross said inductance is to closely approximate the second derivativeof the arrival-curve. T

Connected in parallel with the input circuit of amplifier A is that ofamplifier a the low side of autotransformer 18. Across the .highside ofsaid transformer is connected the input circuit of am lifier Athroughcircuit 19; consequently it will be readily seen in the light ofthe foregoing explanation that the voltage dro across resistance 1' isproportional to t e third derivative of the arrival curve.

Connected in parallel with the in ut circuit of amplifier A is that ofamp ifier a. in the output circuit of which is connected the low side ofautotransformer 20, to the high side of which is connected the inputConsequently the voltage drop across resistance r is proportional to thefourth derivative of the arrival curve. plifiers a, A and b issubstantially propor- The resultant voltage drop across resistances 7*,1' 1' and 1' is thus made sip of the sum'pf individual voltage" dropswhose respectlve waveforms are those of the second, fourth, thlrd andfirst derivatives ofthe arrival curve in predeterminate readily adjustable proportion.

It is obvious that the system shown may be extended to providederivatives of higher orders than the fourth by adding additionalderived circuits. In practice the number of derived circuits requiredwill depend upon, and increase with, the distortion intransformer 16whose low side is connected in series with the output circuit ofamplifier (1 With this arrangement. the voltage drop across the inputcircuit of amplifier'A is substantially the second derivative of thearrival curve since the voltage impressed on the input circuit ofamplifier a is the first derivative thereof. Consequently the voltagedrop across resistance r is proportional to the second derivative of thearrival curve and its magnitude is controllable by either adjusting theamplifier A or adjusting the value of the resistance T Theautotransformer may be replaced by an inductance coil if desired; it ispractically preferable to troduced by the line and the desired speed ofsignaling.

Without addition of a voltage correspondin to the arrival curve itselfor a substantia equivalent thereof, the combination of the derivatives'will give a wave which is not sufficiently sustained. This will beobvious from a consideration of the curves shown in Fig. 3. The branchcircuit 21 leading from the transformer 13 to the amplifier b with itsassociated apparatus supplies this voltage. has a voltagecorrespondingapproximately to the first derivative applied to its input circuit. Itsoutput circuit is connected to a distortion network comprising theresistance 2-2 and'the capacity 23 in parallel. The effect of these isto convert the wave form of the voltage existing across the terminals oftrans- As already indicated the amplifier I).

'in the output circuit of whichis connected as h former 13 into avoltage across said parallel "elements approximating the integral ofthis same as the arrival current itself. This voltage is impressed onthe input circuit of an amplifier A in whose output circuit is connecteda resistance r. Theoretically ne section of such a network could bedesigned to provide a voltage drop across resistance r of practicallythe form desired but a more convenient and exact way of securing thedesired result is to provide successive stages of a network as indicatedon the drawing, applying the output from the successive stages toseparate resistances in circuit -11-12. Thus there are shown inaddition, amplifiers b and b feeding networks 24 and 25 respectively,which latter feed the input circuits of amplifiers A and A respectively.The amplifiers A, A, like A, are each connected to a resistance (r and rrespectively) in the circuit 11-12. The aggregate effect of the voltagesacross resistances r, r and r will then be to add a voltage to thoseacross resistances r to r, substantially like that indicated by curve 1in Fig. 5, which when combined with voltages having wave formscorresponding'to the several derivatives as above described will givethe desired wave form for recording. The results obtained by the systemare dependent upon proper relations among the constants of theapparatus, as will be readil understood, and these must be chosen withue regard to the wave form to be received and the wave form it isdeslred to produce. In order that the output from the amphfiel's A to Ainclusive, may affect the en'- cuit 11--12 in just the proportionsrepresented by the values assigned to the reslstances r to 1" inclusive,it is important that the circuit represented 'by 11-12 be of high tillimpedance. A long transmission line of high impedance wouldsubstantially "satlsfy the requirements. The result may be perfectlyattained, however, if the circuit 1-1--12 is connected to the input sideof a thermionic amplifier (not shown) since no appreciable current willthen flow in circuit and the current emanating from one of theamplifiers A -A cannot affect the potential drop across the resistancesconnected to the other amplifiers.

For the sake of simplicity the amplifying devices have been indicatedmerely by rectangles in Fig. 1, Fig. 2, however, lllustrates thecircuit" represented by the :rectangle in each instance. It comprisesthewell known form of thermionic amplifier 26 having a high resistance27 connected across its input circuit and acondenser 28 in series. Itwill be observed that the capacity 28 and the resistance 27 will, inpractice, be metallicallyconnected with the distortion networks withWhich the amplifiers are associated but they do not constitute any partthereof and inorder'that they may not interfere with the operation ofthe netpreceding circuit.

be large, especially in the circuits dealing.

with t e arrival current or its lower derivatives, in order not toshorten the time period of the applied voltages in these components ofthe signaling current.

In operation the system shown in Fig. 1 may be adjusted to givepractically any desired form of voltage or current in circuit 11-12 bychanging the relative valuesof the difierent components, of the voltagedrops across the resistances in said circuit. This may be, donepartially by adjusting the amplifiers A to A to give varying values tothe components transmitted through them respectively. A very simplemethod of adjusting the wave form, however, is aiforded by theresistances 1' to 1" since upon the value of any one of these dependsthe amount of the corresponding voltage component which is impressed11-12L It is to be observed that the amplifiers in the circuits shownserve not only to compensate for the loss in amplitude due to thecorrecting networks and the attenuation in the line 10, but theyseparate each of the correcting networks from all of the others and thisWithout disturbing its modifying efl'ect upon the Wave form. This is byreason of the fact that the thermionic amplifier is essentially aone-Way device which consumes no appreciable energy in its inputcircuit. This makes it possible to calculate and adjust the. form of thenetwork for giving any one'of the various components as above describedwithout the complications introduced when one network reacts upon itsneighbor.

The modification shown in Fig. 5 differs from that shown in Fig. 1 inthat the circuits for obtaining the successive deriva tives areconnected in series instead of in parallel and a series arrangement forthe integral branch of the circuit is also used in place of the parallelarrangement of Fig.

upon the line 18' and 20' each includes a resistance 26 inseries withthe inductance. The effect of this resistance is to preserve in thesucceeding circuit the component introduced by the The result is thatthe current applied to the resistance r in the circuit 11-12 is acombination of the first, second,' third and fourth derivatives of thearrival current.

The arrangement is simpler than that shown in'Fig. 1 and requires feweramplifiers but it does not have the flexibility of the more elaboratecircuit.

The invention is not' to be regarded as limited to the particular formsherein illustrated and described as it is obvious that variousmodifications may be made without 1. In this case the distortionnetworks 16,

. departing from the spirit of the invention as 2. In a wave-formcorrecting device, means for producmg from the arrival current separatecurrents having wave forms corresponding substantially to the arrivalcurrent itself and one or more of its derivatives respectively, a single.circuit for receiving the several currents so produced, and

, means for determining the relative values of said currents applied tosaid circuit.

3. In a Wave-form correcting device, means for producing from-thearrival current separate currents having wave forms correspondingsubstantially to the arrival current itself and one or more of .itsderivatives respectively, means for amplifying separately thecurrents soproduced, and connections for superlmposing sald amplified currents upona single output circuit in desired proportions.

a. In a wave-form. correcting device,

means for producing from the arrival current a plurality of currentshaving wave forms 0 varying degrees of abruptness and 1 duration,separate circuits into which the .several currents are respectlvelyintroduced, a single outpu t c1rcu1t, and means for 1mposing the severalseparatecurrents upon the output circuit in desired proportions.

5.. In, a wave-form correctlng device, means for producing fromthearrival current a plurality of currents having wave forms of varying.degrees of abruptness and duration, separate circuits into which theseveral currents are respectively introduced, means in each separatecircuit for amplifying the current therein, and means for superimposingthe separate amplified currents upon a single outputcircuit.

6. The method of correcting the wave form of current impulses whichconsists in producing from the arrival current a plurality varyingdegrees of abruptness and duration, separating the several currents intodistinct circuits, and applying the separate currents to a single outputcircuit in desired proportions.

7. he method of correcting the wave form of current impulses whichconsists in producing from the arrival current a plurality of currentshaving wave forms of of currents having wave forms ofvarying degrees ofabruptness and duration separating the several currents into 'dlStlIlCtcircuits, amplifying the currents separately and applying the separatecurrents to a single output circuit in desired proportions.

8. The method of correcting the wave form of current impulses whichconsists in producing from the arrival current a plurality of separatecurrents corresponding substantially in wave form to the successivederivatives of the arrival current, and combining in a single outputcircuit the currents so produced with a current whose wave formcorresponds substantially with the arrival current itself.

9. The method of correcting the wave form of current impulses whichconsists in producing from the arrival current a plurality of currentsof varying wave forms, separating each of the currents from the othersto prevent reaction of one upon another, amplifying the producedcurrents separately to desired strength, and superimposing the separatecurrents in a single output circuit in a proportion to give the desiredresulting wave form.

'10. In a signal receivin circuit, an inductive device in which t earriving impulses are received, parallel circuits connected to saidinductive device one of which is provided with means for producing acurrent having awave form approximatin the wave form of the arrivalcurrent, an another of which is provided with means for producingcurrents having wave forms corresponding substantially to derivatives ofsaid arrival current, and means forimposing the currents in saidparallel circuits upon a single output circuit in desired proportions.

'11. In a signal receiving circuit, an inductive device in which thearriving impulses are received, parallel circuits connected to saidinductive device, means located in certain of said parallel circuits forproducing in them separately, current impulses of different slopes andduration but all more sustained than the impulses taken from theinductive device, and means in the other parallel circuits for producinin them separately current impulses of different slopes and duration butall more abrupt than the impulses taken from the inductive device, andmeans for imposing currents from the parallel'circuits and saidinductive device upon a single output circuit in desired proportions.

In testimony whereof, I have signed my name to this specification this2nd day of July, 1918.

JOHN R. CARSON.

